1. Field of the Invention
The present invention relates to a measuring device for an arrayed-waveguide diffraction grating, and in particular, to a measuring device for an arrayed-waveguide diffraction grating that demultiplexes optical signals of different wavelengths.
This application in based on patent application No. Hei 11-205423 filed in Japan, the content of which is incorporated herein by reference.
2. Description of the Related Art
Wavelength multiplex communications in which optical signals of multiple different wavelengths are multiplexed and sent in one optical fiber have entered practical application in recent years. An optical multiplexer/demultiplexer, which multiplexes and demultiplexes the light according to its wavelength, is one important element in this type of communications system.
It is conventionally known that bulk diffraction gratings, dielectric multilayers and the like may be used for this optical multiplexer/demultiplexer. However, these conventional devices have a variety of defects. Namely, the selected wavelength is difficult to set, the devices are expensive because the manufacturing steps are complicated, and they experience considerable loss. Thus, the application of these conventional devices to wavelength multiplex communications in which multiple wavelengths are multiplexed/demultiplexed is difficult.
Accordingly, in recent years, attention has been given to arrayed-waveguide diffraction gratings such as recorded in xe2x80x9cCompilation 1, C-3, p. 162 of lectures given at the 1996 meetings of the electronics society of the Institute of Electronics, Information and Communication Engineers of Japanxe2x80x9d.
FIG. 3 is a planar view showing an example of an arrayed-waveguide diffraction grating.
As shown in FIG. 3, an arrayed-waveguide diffraction grating consists of a plurality of input waveguides 50; an input-side slab waveguide 52 onto which optical signals from input waveguides 50 are incident; an arrayed-waveguide 54 consisting of a plurality of waveguides attached to the opposite end of the input-side slab waveguide 52; an output-side slab waveguide 56 attached to the other end of arrayed-waveguide 54; and a plurality of output waveguides 58 attached to the other end of the output-side slab waveguide 56.
Optical signals being incident from input waveguides 50 are incident on the input-side slab waveguide 52, and then are incident with the same phase on arrayed-waveguide 54 consisting of a plurality of waveguides.
The input terminal of arrayed-waveguide 54 and the output terminal of input waveguides 50 are each disposed on respective circles. The radius of the circle on which the input terminal of arrayed-waveguide 54 is disposed is two-fold greater than the radius of the circle on which the output terminal of input waveguides 50 is disposed. The center of the circle on which the input terminal of arrayed-waveguide 54 is disposed is located on the circle on which the output terminal of input waveguides 50 is disposed.
Each of the waveguides in arrayed-waveguide 54 is adjusted so as to provide an equivalent interval phase difference. An output-side slab waveguide 56 is disposed to the other end of arrayed-waveguide 54. With regard to the disposition of arrayed-waveguide 54, output-side slab waveguide 56, and output waveguide 58, the output terminal of arrayed-waveguide 54 and the input terminal of output waveguide 58 are each disposed on respective circles, as was the case on the input side. The radius of the circle on which the output terminal of arrayed-waveguide 54 is disposed is two-fold greater than the radius of the circle on which the input terminal of output waveguide 58 is disposed. The center of the circle on which the output terminal of arrayed-waveguide 54 is disposed is located on the circle on which the input terminal of output waveguide 58 is disposed.
Cross-talk is one performance indicator for this arrayed-waveguide diffraction grating. This cross-talk is defined as the ratio of the optical power of a wavelength with respect to the optical power of the wavelength which is to be selected. Cross-talk expresses the signal spill-over between channels.
It is necessary to realize a low level of cross-talk in order to achieve high-quality communications. In order to realize a low level of cross-talk, the length of the optical path in arrayed-waveguide 54 (i.e., the product of length and the refractive index) needs to be controlled with an accuracy that is on the order of {fraction (1/10)} of the wavelength. As a result, accurate measurement of the optical path length and trimming thereof based on the results of this measurement are required.
It is known that a method employing a Mach-Zehnder interference optical system and Fourier transform demultiplexing method as disclosed in K. Takada, H. Yamada, Y. Inoue, Optical Low Coherence Method for Characterizing Silica-Based Arrayed-Waveguide, Journal of Lightware Technology, Vol. 14, No. 7., p. 1677, 1996, can be used as a method for measuring the optical path length in arrayed-waveguide 54.
FIG. 4 is a block diagram showing the structure of the conventional measuring device for an arrayed-waveguide diffraction grating.
The numeral 100 in FIG. 4 indicates a LED (light emitting diode) that radiates light of a sufficiently short coherent length having a wavelength of 1.5 xcexcm. 102 is a LD (laser diode) that radiates light of a sufficiently long coherent length having a wavelength of 1.3 xcexcm.
104 is an optical coupler that multiplexes the light radiated from LED 100 and LD 102, splits the light into equal intensities, and then radiates it from each of two radiating terminals. An arrayed-waveguide diffraction grating 106, which is to be measured, is connected to one radiating terminal of the optical coupler, and an optical path length varying device 108 for changing the optical path length is connected to the other radiating terminal. 110 is an optical coupler which is identical to optical coupler 104. The output terminal of arrayed-waveguide diffraction grating 106 and the output terminal of optical path length varying device 108 are connected to respective input terminals of optical coupler 110.
The above-described optical coupler 104, arrayed-waveguide diffraction grating 106, optical path length varying device 108, and optical coupler 110 form a Mach-Zehnder interference optics system.
Numeral 112 is a wavelength demultiplexer which is connected to one output terminal of optical coupler 110, and which radiates input light from different radiating terminals at each wavelength.
Optical detector 116 is connected to one of the radiating terminals of wavelength demultiplexer 112 via optical fiber 114. Optical detector 116 converts the incident optical signal to an electric signal and outputs this result. The output electric signal is output to waveform recording device 120 via signal line 118.
Optical detector 124 is connected to the other radiating terminal of wavelength demultiplexer 112 via optical fiber 122. Like optical detector 116, optical detector 124 converts the incident optical signal to an electric signal and then outputs this result. The electric signal output from optical detector 124 is input to clock generator 128 via signal line 126. Clock generator 128 outputs a frequency clock in response to the value of the electric signal that is input. Note that clock generator 128 is connected to waveform recording device 120 by signal line 130. It is also possible to record the output of clock generator 128.
The principle for measuring the optical path length of arrayed-waveguide diffraction grating 106 in the aforementioned design is as follows. Namely, light having a wavelength of 1.5 xcexcm that was radiated from LED 100 is incident on the Mach-Zehnder interference optics system via optical coupler 104. When this light is incident on the Mach-Zehnder interference optics system, then 1.5 xcexcm light which has propagated through the arrayed-waveguide diffraction grating 106 inside the Mach-Zehnder interference optics system, and the 1.5 xcexcm light which has propagated through optical path length varying device 108 are coupled by optical coupler 110. The coupled signal is incident on optical detector 116 via wavelength demultiplexer 112 and optical fiber 114, and is converted to an electric signal.
Isolated beat signals are generated because of mutual interference between the coupled light. As a result, it is possible to determine the correct optical path length for the arrayed-waveguide by performing a Fourier transform on these beat signals.
In a conventional measuring device for an arrayed-waveguide diffraction grating, it is necessary to accurately measure an optical path length of 1.55 xcexcm when moving optical path length varying device 108 at roughly a constant speed, i.e., when decreasing the optical path length at a constant proportion. The method for accurately measuring the length of the optical path for light having a wavelength of 1.5 xcexcm will now be explained.
When light of 1.3 xcexcm wavelength having a sufficiently long coherent length is radiated from LD 102 and is incident on the Mach-Zehnder interference optics system in FIG. 4, then the light which has propagated through each arrayed-waveguide inside the Mach-Zehnder interference optics system and the light which has propagated through optical path length varying device 108 are coupled. Isolated beat signals are generated due to interference between 1.3 xcexcm wavelength light which has propagated through arrayed-waveguide diffraction grating 106 and the 1.3 xcexcm wavelength light which has propagated through optical path length varying device 108. This beat signal is incident on optical detector 124 via wavelength demultiplexer 112 and optical fiber 122, and is converted to an electric signal. The converted electric signal is input to clock generator 128 via signal line 126, converted to information for changing the optical path length of 1.5 xcexcm wavelength light by on clock generator 128, and is recorded in waveform recording device 120.
Next, the method for converting the beat signal for 1.3 xcexcm wavelength light to information for varying the optical path length of 1.5 xcexcm wavelength light will now be explained.
When the intensity of 1.3 xcexcm wavelength light is denoted by P1, the intensity of 1.3 xcexcm wavelength light is denoted by P2, the wavelength of 1.3 xcexcm wavelength light is denoted by xcex, the ratio of each of these light intensities is denoted by r=P1/P2, and the optical path length of 1.5 xcexcm wavelength light is denoted by L, then the beat signal B (L) of 1.3 xcexcm wavelength light generated by the device in FIG. 4 can be expressed by the following equation.       B    ⁡          (      L      )        =            1      2        ⁡          [                                                  (                              1                -                                  r                                            )                        2                                1            +            r                          +                                            4              ⁢                              r                                                    1              +              r                                ⁢                                    cos              2                        ⁡                          (                                                π                  ⁢                                      xe2x80x83                                    ⁢                  L                                                  2                  ⁢                  λ                                            )                                          ]      
Based on the preceding equation, by varying the shift length, i.e., the optical path length L with respect to 1.5 xcexcm wavelength light, of optical path length varying device 108, then the amplitude in the above equation changes in a cyclical manner.
Thus, optical path length L of 1.5 xcexcm wavelength light can be accurately measured to a resolution of a wavelength that is half that of 1.3 xcexcm wavelength i.e. light wavelength of xcex/2, by converting the beat signal of 1.3 xcexcm wavelength light to an electric signal at optical detector 124 in the 1.3 xcexcm wavelength band, and detecting the maximum and minimum values of the amplitude of the beat signal of 1.3 xcexcm wavelength light at clock generator 128.
In order to detect the maximum and minimum values of the amplitude of the beat signal of the 1.33 xcexcm wavelength light, it is clear that a larger amplitude for the beat signal is preferred. This condition is met when r=1, i.e., when P1=P2.
In the above-described device, light of 1.3 xcexcm wavelength which has passed through the waveguide in arrayed-waveguide 54 which is to be measured experiences loss at the arrayed-waveguide diffraction grating which is to be measured and coupling loss between waveguides due to the alignment of the optical axis during measurement. As a result, the intensity of the light is weaker than the light which passed through optical path length varying device 108, i.e., the AMP value is smaller. Thus, problems occur such that the amplitude of the beat signal of the 1.3 xcexcm wavelength light cannot be obtained, the amplitude maximum and minimum values cannot be detected by clock generator 128, and the optical path length of L 1.5 xcexcm wavelength light cannot be accurately measured.
The arrayed-waveguide diffraction grating 106 that is to be measured is a wavelength demultiplexer. Thus, when the wavelength of the 1.3 xcexcm wavelength light and the transmission band do not conform, the light intensity is significantly attenuated. A beat signal of a sufficient amplitude cannot be obtained, the maximum and minimum values of the amplitude cannot be detected at clock generator 128, and optical path length L of 1.5 xcexcm wavelength light cannot be accurately measured.
With regard to light having a 1.3 xcexcm wavelength that has passed through the waveguide in arrayed-waveguide 54 that is to be measured, loss in arrayed-waveguide diffraction grating 106 which is the subject of measurement will differ for each element, and the coupling loss between fiber waveguides will differ for every measurement depending on the alignment of the optical axis at this time. As a result, the intensity ratio of the light which has passed through optical path length varying device 108 is not constant, i.e., the AMP value is not constant. Thus, various problems occur, i.e., a 1.3 xcexcm beat signal of sufficient amplitude is obtained or not obtained, the maximum value and minimum value of the amplitude can or cannot be detected at the clock generator 128, etc. As a result, the optical path length L with respect to light of wavelength 1.5 xcexcm cannot be accurately measured.
An example of the results obtained from measurements using a conventional device will be used to explain the aforementioned problems in detail.
FIG. 5 shows an example of the results obtained when measurements are carried out using a conventional measuring device for an arrayed-waveguide diffraction grating.
LED 100 is employed as the light source for radiating light of 1.5 xcexcm wavelength having a sufficiently short coherent length, and an LD 102 for communications use is employed as the light source for radiating light of 1.3 xcexcm wavelength having a sufficiently short coherent length.
A 3 dB fiber coupler was employed for optical coupler 104,110, while a design was employed for optical path length varying device 108 in which a prism and a reflecting mirror are installed on a linear stage. Wavelength demultiplexer 112 employs a 1.5 xcexcm/1.3 xcexcm demultiplexing fiber coupler. Optical detectors 116,124 are realized using a design in which the analog-out output of a light power meter is amplified using an electric amplifier and is converted to digital by an A/D converter. A PC (personal computer) is employed for waveform recording device 120. The results obtained from this design when the beat waveform of an arrayed-waveguide diffraction grating 106 in which there are 40 waveguides is measured are shown in FIG. 5.
The amplitude of the beat waveform is on the vertical axis in FIG. 5 (the units are optional and have been normalized in the dynamic range of the A/D converter). The horizontal axis is the sampling clock for the A/D converter when optical path length varying device 108 is shifted at an approximately constant speed.
Note that the ratio between intensity P1 of the light which passed through the waveguide path which is to be measured and intensity P2 of the light which passed through optical path length varying device 108 is 2000:1. As shown in FIG. 5, the signal is buried in optical noise and electric noise during electric conversion, and the maximum and minimum values of the amplitude cannot be detected by clock generator 128. Thus, it is not possible to accurately measure the optical path length L with respect to light of wavelength 1.5 xcexcm at a resolution of optical wavelength xcex/2 for light of wavelength 1.3 xcexcm.
The present invention was conceived in view of the above-described circumstances and has as its objective the provision of a measuring device for an arrayed-waveguide diffraction grating which can measure an accurate optical path length for light of wavelength 1.5 xcexcm in the arrayed-waveguide diffraction grating.
In order to resolve the above problems, the present invention is a measuring device for an arrayed-waveguide diffraction grating provided with a first light source; a first optical path, onto which light radiated from the first light source is introduced and on which the arrayed-waveguide diffraction grating to be measured is disposed; a second optical path, on which an optical path length varying means is disposed; and an interference means for causing interference in light that has passed through the first optical path and the second optical path; this measuring device for an arrayed-waveguide diffraction grating being characterized in the provision of
a second light source;
a first multiplexing means for multiplexing light radiated from the second light source to a stage preceding the optical path length varying means;
a demultiplexing means for demultiplexing the second light source provided to a stage proceeding the optical path length varying means and coupled to the second optical path;
an attenuating means for attenuating light radiated from the second light source by a specific amount; and
a second multiplexing means for multiplexing light radiated from the demultiplexing means and the attenuating means.
The present invention is further characterized in that the attenuating means attenuates light radiated from the second light source so that it has an intensity that is 1xcx9c100 fold greater than the light radiated from the first demultiplexing means.
The present invention is further characterized in that the wavelength of the light radiated from the first light source is 1.5 xcexcm, and the wavelength of light radiated from the second light source is 1.3 xcexcm.